Honestly, most post-Hogwarts growth fics stick to the seventh year or the immediate aftermath. The ones where he leaves earlier are trickier to find, but they exist if you dig through the archives. 'The Denarian Renegade' and its sequels by Shezza come to mind—Harry gets expelled in third year after the dementor incident on the train, and it spirals into a crossover with 'The Dresden Files' universe. His growth is less about mastering spells and more about surviving in a much darker, grittier magical underworld. It's not for everyone; the tone shifts dramatically, and the power scaling gets pretty intense.
Another one I half-remember is 'What You Leave Behind' by Newcomb, where he walks away after fifth year. The focus is on him building a life completely separate from the wizarding world, learning non-magical trades, and dealing with the psychological fallout without a epic war plot forcing him back. The growth feels quieter, more internal, which I found refreshing after so many fics where he just becomes an overpowered auror. The pacing can drag in the middle, though, and it's been years since I checked if it was ever finished.
Those are the two that left a real impression. You might have better luck searching for 'Harry leaves Hogwarts early' or 'AU no Hogwarts' on AO3 or fanfiction.net, filtering for complete works over 100k words.